Kevin T. Chu

Continuum Modeling of Ion Transport in Nano-electrochemical Systems

APS March Meeting,
Montreal, Canada,
March 25, 2004

Authors

Kevin T. Chu (MIT, Mathematics)

Martin Z. Bazant (MIT, Mathematics)

Abstract

Traditional models of macroscopic electrochemical systems are based on
two fundamental assumptions: (i) electroneutrality of the bulk
solution and (ii) Boltzmann equilibrium of diffuse charge in the
interfacial double layers. In nanostructures, these assumptions break
down as the distinction between ``bulk'' and ``interface'' becomes
blurred. Moreover, at the nanoscale, tiny voltages can lead to
enormous electric fields, which can drastically alter diffuse-charge
distributions and current-voltage relations. Although quantum
mechanical descriptions are important the atomic scale, continuum
models may still provide valuable analytical insights at the nanoscale
(as in the classical theory of the double layer), albeit in a
different limit from macroscopic electrochemistry. Here, we study the
classical Poisson-Nernst-Planck equations in nano-electrochemical
systems (near the scale of the screening length), including the
boundary conditions for Faradaic reactions and surface
capacitance. For binary electrolytes, our analysis reveals new
non-equilibrium double-layer structures near and above the classical
diffusion-limited current and polarographic (V vs. I) curves that are
very sensitive to interfacial properties. We also consider the exotic
case of a single unscreened charge carrier to model lithium diffusion
in SiO2 nano-thin films, which has recently been demonstrated in
on-chip micro-batteries.